As a storage medium of information (data) such as a document, an image and a sound, storage media such as a hand disk (HD), a digital video disk (DVD), a magneto-optical disk (MO), a compact disk (CD) and a laser disk (LD) have been developed and the capacity thereof have been increased.
When data is reproduced from a medium such as an optical disk in which data is stored, it is necessary to digitize a signal read from the medium into information of a bit string comprising "1s" and "0s" (digital). FIG. 1 is a diagram showing a method (principle) of digitizing a signal read from a medium which is recorded with PWM scheme. In FIG. 1, 1 shows a signal read from the medium and 2 shows a slice level signal (hereinafter referred to as "slice signal") for digitizing into a binary value. The slice signal is normally obtained as an average of an integration of an output signal from the medium by a digital sum value circuit (DSV) which has an integration circuit. 3 shows a clock for digitizing into a binary value and is hereinafter called a "time cell" in this specification. The time cell 3 is normally determined and controlled by a phase lock loop (PLL) based on the frequency of the output signal from the medium. The time cell 3 has a fixed phase -.pi. to +.pi. as shown by a reference number 4.
In FIG. 1, a time cell containing an intersection 6 of the read signal and the slice signal 2 is a bit "1" while a time cell which does not contain an intersection is a bit "0". As such, the read signal is converted into bit string information comprising "1" and "0" as shown by the reference number 7 in FIG. 1.
The reproduced bit string information varies due to the variation of the slice level of the slice signal 2 and the width (period) of the time cell. For example, when the slice level of the slice signal is shifted upward from a position 8 where the level is supposed to originally exist to a position 9 shown by a dotted line as shown in FIG. 2, the position of the intersection with the read signal 1 is shifted forward and backward as shown by the reference number 10 and 11. As a result, the time cell in which an intersection should be contained is shifted from the position where the cell should originally exist.
Further, an error is included in the intersection between the read signal and the slice signal due to the influence of a distortion of a signal waveform by a noise. FIG. 3 is a diagram showing the influence of a variation of the slice level and a phase error (variation) of a measured value within a single time cell. In FIG. 3, the dotted line 12 indicates a shift of the phase value due to the variation of the slice level. The 3 measured values A, B and C in the intersection each has an error width 13 of the measured value. Incidentally, a correction of polarity is added to each phase error so that the error is shown only with a positive value in FIG. 3. The measured value C may shifts to the adjacent time cell beyond the phase +.pi. due to an error. In other words, the time cell in which the intersection should be contained is probable to be shifted. Such shift of the time cell containing the intersection similarly occurs when the width (period) of the time cell becomes larger or smaller. As a result, it is probable that the reproduced bit string information is different from the true original data.
The reproduced bit string information is then checked by an error correction circuit (ECC) to correct the error. However, the correction is not necessarily perfect. Accordingly, it would be advantageous to effect a correction corresponding to the error correction of the reproduced bit string information by some different method.
Bit string information which reproduces an "ID" and the like existing in a pre-formatted portion of an optical disk can not be corrected by an ECC. Therefore, it would be very advantageous to correct such data which can not be corrected by an ECC so as to precisely reproduce it. This is because "SM" and "VFO", etc., are normally preceding to an area (sector) where data is stored so that stored data can not be reproduced or written sector by sector unless a precise reproduction is effected.